Polyvinyl acetal resins are the reaction products of polyvinyl alcohol and aldehydes. The most common polyvinyl acetal resins are derived from normal aldehydes such as acetaldehyde and n-butyraldehyde. Commercially, the most important polyvinyl acetal resin is polyvinyl butyral, which has been used for decades as an interlayer for automotive safety glass.
Polyvinyl butyral is the resin of choice for safety glass because of its overall balance of superior properties, optical clarity, and low cost. The lack of additional functionality in the aldehyde component, however, is somewhat limiting. It would be desirable for some applications to have primary hydroxyl groups on the acetal resin, groups that could promote interlayer interaction or be chemically modified to achieve other desirable properties.
Aliphatic esters such as dihexyl adipate, dibutyl sebacate, and tetraethylene glycol diheptanoate, are commonly added (usually at about 30-40 wt. %) to plasticize polyvinyl butyral. Even distribution of the plasticizer may be difficult to achieve; under various temperature extremes, a plasticizer may tend to phase separate or leach out of the polyvinyl acetal resin. Thus, it is desirable to develop plasticized polyvinyl acetal compositions that resist phase separation. In addition, added plasticizers generally reduce the overall physical properties of the polyvinyl acetal resins.
Surprisingly little is known about polyvinyl acetal resins derived from hydroxy-substituted aldehydes. Pawlowski (U.S. Pat. No. 4,940,646) teaches polyvinyl acetal compositions derived from hydroxy-substituted aldehydes. The products are useful binders for photosensitive compositions. Yoshioka et al. (Japanese Patent Application Kokai No. 32-8470) teach a method for treating synthetic fibers of polyvinyl alcohol with hydroxyaldehydes under acid conditions to make treated fibers having acetal functionality on their surface. Neither reference teaches plasticized polyvinyl acetal compositions useful for laminar structures.
Still needed in the art are polyvinyl acetal resins having improved physical properties. Resins that can be modified chemically to vary properties, and plasticized resins that resist phase separation are also needed.